Development and differentiation of the embryo must in large part be controlled by gene activity prior to and during embryogenesis. To study the process of activation and deactivation it is necessary to use a gene system that is modulated during development and is amenable to biochemical analysis. Histone genes are activated during oogenesis, and then after fertilization, in a unique manner. At the mid-blastula stage (12 hours of development) there is an abrupt change in the histone mRNA synthesis pattern. Over a period of several hours the early histone mRNAs are gradually replaced on the polysomes by a new class of histone mRNAs. This new class, the late histone mRNAs, differs in both size and sequence from the early mRNAs and are coded for by a new (late) gene class. Experiments in this proposal, utilizing recombinant DNA technology, will examine, in detail, the sequence structure of maternal, early and late histone mRNAs and the organization of the genes or gene clusters coding for these mRNAs. Furthermore, experiments are designed which will analyze in vivo and in vitro the precursor transcripts of histone genes utilizing sea urchin nucleii as the first step in setting up a means for studying histone gene control.